Field
The present invention relates to a light guide plate and a surface illumination device. More specifically the present invention relates to a surface illumination device used as a backlight in a liquid crystal display device, and a light guide plate used in said surface illumination device.
Related Art
As mobile devices with built-in surface illumination devices become thinner, the surface illumination devices are required to have increasingly thinner profiles. Consequently, the light guide plate must also have a thinner profile to create a thinner surface illumination device. Despite that, even if a planar light guide plate were given a thin profile, there are limitations to reducing the height of the LED in the light source. Therefore, when using a planar light guide plate, the height of the light source will be greater than the thickness of an end surface (light input surface) of the light guide plate. Thus, the light source, which is arranged opposite the light input surface of the light guide plate protrudes from above the upper surface of the light guide plate. In this manner when the light source protrudes from above than the light guide plate, a portion of the light emitted from the light source leaks externally without all the light entering the light input surface of the light guide plate, reducing the light use efficiency in the light guide plate.
The surface illumination device 11 illustrated in FIG. 1 is traditionally used to address this disadvantage. A light source 12 is arranged facing a light input surface 17 of a light guide plate 13 (described next) in a surface illumination device 11. The light guide plate 13 is provided with a light conducting portion 14 that is thicker than the main light guiding body 15 and arranged on an end of the planar main light guiding body 15. The light conducting portion 14 is provided with a slanted surface 16 that is inclined from the thickest section of the light conducting portion 14 toward an end of the main light guiding body 15.
The light input surface 17 provided on the end surface of the light conducting portion 14 higher than the light emission plane (light output window) of the light source 12 in the surface illumination device 11 illustrated in FIG. 1. Therefore, the light emitted from the light source 12 can be efficiently taken into the light conducting portion 14. Additionally, the light incident on the slanted surface 16 while traveling from the thicker light conducting portion 14 toward the thinner main light guiding body 15 is reflected at the slanted surface 16 and guided to the main light guiding body 15. The light guided to the main light guiding body 15 is reflected by an optical pattern (not shown) provided on the lower surface of the main light guiding body 15 and is output externally from a light output surface 18 provided on the upper surface of the main light guiding body 15. Thus, in such a surface illumination device 11, the light from the light source 12 can be efficiently taken in from the light input surface 17, while reducing the amount of light leaked from the light guide plate 13 as the light from the light source is transmitted from the light conducting portion 14 to the main light guiding body 15. Therefore, the light use efficiency of the surface illumination device 11 improves. Furthermore, Light tends not to leak from the slanted surface 16 and the edges of the upper surface of the main light guiding body 15. Therefore, the leaked light tends not to produce glare at the edges of the light output surface 18, and the quality of the surface illumination device 11 improves.
However, as the demand increases for higher-quality surface illumination devices, even the improvements to the light use efficiency in the surface illumination device 11 illustrated in FIG. 1 become less adequate. In particular, the light output from the light source 12 towards the front direction has a small angle of incidence relative to the slanted surface 16; therefore, the light emitted towards the front direction passes through the slanted surface 16 and tends to leak externally from the light guide plate 13. Consequently, it is not possible to obtain sufficient light use efficiency.
“A Novel LED-Backlight System with Tilted Cylindrical Surfaces on the Light Guide Plate” by Kazutada Takaira, Yasuhiro Morii, Seiji Sakai, Kenji Itoga, Akimasa Yuuki; SID Symposium Digest of Technical Papers, Volume 44, Issue 1, pages 820-823, June 2013 (http://onlinelibrary.wiley.com/doi/10.1002/j.2168-0159) (hereinafter, “cited reference”) proposes a different surface illumination device structure that improves the light use efficiency in a device having the structure illustrated in FIG. 1. FIG. 2A is a perspective view illustrating the structure of a surface illumination device 21 disclosed in the cited reference. The surface illumination device 21 is provided with two cylindrical surfaces 22 in the slanted surface 16 of the light conducting portion 14. The two cylindrical surfaces 22 create a V-shaped groove 23 therebetween. The two cylindrical surfaces 22 pass through the base line of the groove 23 and are symmetrical about a plane perpendicular to the light input surface 17 and the light output surface 18. Each axis center of the cylindrical surfaces 22 also inclines closer to the light input surface while moving up the axis center. The inclination of the cylindrical surfaces 22 is equal to the inclination of the slanted surface 16. Accordingly, the axis center of the cylindrical surfaces 22 is parallel to the slanted surface 16. The principle axis (the axis coinciding with a principle light beam) through the light emission center of the light source 12 passes through a line intersecting both cylindrical surfaces 22 (the base line of the groove 23) when viewed from a direction orthogonal to the upper surface of the main light guiding body 15, and is located on a straight line orthogonal to the light input surface 17.
FIG. 2B is a schematic plan view illustrating near the light source of the surface illumination device 21 depicted in FIG. 2A. In the surface illumination device 21, the slanted surface 16 is provided before providing the cylindrical surfaces 22. Further, the light rays output from the light source 12 in a front direction are incident on the cylindrical surfaces 22 at an incident angle larger than the incident angle on the slanted surface 16 as depicted by the arrows in FIG. 2B. Therefore, the light traveling in a front direction tends reflect at the cylindrical surfaces 22. As a result, less light leaks from a surface of the light conducting portion 14 along the front direction of the light source 12, and the light use efficiency improves.
FIG. 3 is a graph comparing the light use efficiency of each of the surface illumination devices 11 and 21 in FIG. 1 (Sample 1) and in FIG. 2A (Sample 2) respectively. In FIG. 3, the horizontal axis represents a gradation ratio t/T, and the vertical axis represents a light use efficiency R. The gradation ratio represents the proportion t/T of the height t of the main light guiding body 15 to the height T of the light conducting portion 14. The light use efficiency R is a percentage (%) of the amount of light incident on the light guide plate 13 that is emitted from the light output surface 18 of the light guide plate 13. More specifically, as illustrated in FIG. 4, an optical detector 24 was arranged facing the end surface of the light guide plate 13 that is opposite the light input surface 17. The light use efficiency R in FIG. 3 was calculated using (Is/Io)×100 (%), with To representing the light intensity of the light exiting the light source 12 and incident on the light input surface 17, and Is representing the light intensity measured by the optical detector 24 (i.e., the light intensity of the light exiting the end surface opposite the light input surface 17). However, the measurements taken and depicted in FIG. 3 were of light guide plates having no optical patterns to cause the light to exit from the light output surface 18. That is, the light guide plates were flat, with smooth upper and lower surfaces in the main light guiding body. Therefore the amount of light measured by the optical detector 24 was the amount of light that would have been output upward from the effective area of the light output surface 18 after the light was reflected by an optical pattern.
As illustrated in FIG. 3, the light use efficiency R of the surface illumination device 21 in FIG. 2A (Sample 2) improves significantly compared to the surface illumination device 11 in FIG. 1 (Sample 1). Incidentally, as can be ascertained from the state of the light rays illustrated in FIG. 2B, light leakage toward the front of the light source 12 can be prevented in the case of the surface illumination device 21 which includes the cylindrical surfaces 22. However, the light output from the light source 12 toward the front direction spreads transversely due to being reflected at the cylindrical surfaces 22. As a result, the emission luminance of the light output surface 18 along the front direction of the light source 12 deteriorates, introducing an uneven luminance.
To ensure that the emission luminance of the (effective area of the) light output surface 18 is uniform, conceivably, a lenticular lens 32 may be provided on the upper surface of the main light guiding body 15 in a surface illumination device 31 as illustrated in FIG. 5. A lenticular lens 32 provided on the light output surface 18 alters the directivity of the light output from the light output surface causing the light to spread in the transverse direction, and an even emission luminance in the light output surface 18.
Despite that, when the lenticular lens 32 is provided on the light output surface 18, light tends to leak from the lenticular lens 32, thus creating a diagonal bright line on the upper surface of the main light guiding body 15.
The above situation is described using FIGS. 6A, 6B, and 6C. FIG. 6A shows the state of the surface illumination device 21 with no lenticular lens, and FIG. 6B shows the state of the surface illumination device 31 in FIG. 5 having the lenticular lens 32. In each case, the directivity of light at the end surface opposite the light input surface 17 of the main light guiding body 15 was observed from a direction orthogonal to the light input surface 17. In FIGS. 6A and 6B, a portion is darker in accordance with the light intensity. FIG. 6C illustrates the results of arranging an optical detector 25 opposite the light output surface 18 of the main light guiding body 15 as illustrated in FIG. 4, and using the optical detector 25 to measure the amount of light leaked from the light output surface 18 of the surface illumination device 31. In FIG. 6C, a portion is lighter in accordance with the amount of light leaked.
FIG. 6A shows light beams of obliquely-directed light 34 spreading transversely around the light beams of a front-directed light 33 oriented substantially orthogonal to the light input surface 17 (i.e., frontward). The obliquely-directed light 34 spread transversely due to the cylindrical surfaces 22. Whereas, in FIG. 6B only the front-directed light 33 remains noticeable, while the obliquely-directed light 34 disappears. That is, the obliquely-directed light 34 appears as shown in FIG. 6A, and the same obliquely-directed light 34 disappears from FIG. 6B because this light leaks from the lenticular lens 32. In this manner, the light leaked from the light output surface 18 due to the lenticular lens 32 on the surface illumination device 31 in FIG. 5 becomes the diagonal bright lines illustrated in FIG. 6C (the direction represented by the thin arrows). The light guide plate 13 used here had no optical pattern thereon; therefore, although in this case light should not have leaked from the upper surface of the main light guiding body 15 by design, the light leaking from the lenticular lens 32 created the diagonal bright lines.
The surface illumination device 31 is essentially the surface illumination device 21 illustrated in FIG. 2A with a lenticular lens 32 added thereto. Although light spreads transversely from the cylindrical surfaces 22 in the surface illumination device 31, the light leaks from the lenticular lens 32 and reduces the light use efficiency in the surface illumination device 31. Sample 3, represented by dotted lines in FIG. 3, shows the results of measuring the light use efficiency of the surface illumination device 31 in FIG. 5. As can be understood from comparing Sample 2 and Sample 3, the light use efficiency decreases significantly when a lenticular lens 32 is added to the surface illumination device 21 in FIG. 2A.
Given a surface illumination device with a pair of cylindrical surfaces in the slanted surface of the light guide plate, a lenticular lens may be added to the upper surface of the main light guiding body to address light uniformity problems. However, based on the above technical background, there is a need to prevent the reduction in light use efficiency and the generation of bright lines that occur when adding the lenticular lens.